1. Field of the Invention
The present invention relates to an image reading apparatus used in an apparatus for electrically processing an image.
2. Related Background Art
In order to electrically process an image, an image reading apparatus is used to photoelectrically read an original image by using an image sensor such as a CCD sensor and forming an image signal representing an original image.
In such an image reading apparatus, an MTF (Modulation Transfer Function) as one of the factors for accurately reading an original image must be improved.
An improvement in the MTF in a conventional image reading apparatus is achieved by arranging an optical lens for reading an image to close to an ideal lens or by decreasing an opening of the image sensor.
Even in an ideal lens, a decrease in MTF is theoretically present. It is difficult to accurately read a thin line having a high spatial frequency. A decrease in opening size of the image sensor is equal to a decrease in area of a light-receiving element used for photoelectric conversion in the image sensor. As a result, the photoelectrically converted electrical signal has a small magnitude, and electrical processing of this signal is very difficult.
Another conventional image reading apparatus is a color image reading apparatus for causing an optical filter to perform color separation of red (R), green (G), and blue (B) of a color original image, and photoelectrically converting the resultant color images into electrical signals. However, MTFs for the R, G, and B may often be different from each other. When MTFs are different and a color image is reproduced using the read electrical signals, color unbalance occurs, and accurate color image reproduction cannot often be performed.